Automatic volume control circuits



Jan. 1, 1935. w. L. CARLSON 1,985,914

AUTOMATIC VOLUME CONTROL CIRC UIT'S Filed Aug. 29, 1930 AMPL/F/GAT/O/V lI I 1 1 l I AMPLIFIER GRID BIAS POTENTIAL Inventor: Wendell L/Carilson,

by MW His Attorney.

Patented Jan. l, 1935 UNITED STATES 1,985,914 "J AUTOMATIC VOLUME EGQNTROL CIRCUITS Wendell Carlsom'HaddonfiellljhUJ; a'ssi gnor to GeneralElectric Company,; corporation of New York Application August 29, 1930,serial No. mist Claims. (c1.,250 20)' The present invention relates toradio receiving; apparatus embodying hot-cathode, grid-controlledelectric discharge devices commonly known as vacuum tubes, arranged toamplify and detect electric signals, such as.:modulated radio waves, andhas for its object toprovide an improved circuit arrangement inconnection withan amplifier and detector in suchapparatus, whereby itmay operate automaticallwto deliver substantially. a constant signal.output in response to i a signal input of varying strength. y. 5

, .In accordance with the invention, advantage is taken of the fact thatthe'response. or amplification characteristic of an electric dischargeamplifier may be varied between wide limits by slight changesin thebiasing potential: applied to the control electrode or .grid circuitthereof when operating such :circuit in the positive range of biasingpotentials, and of the fact as 'is well known,;that the anode currentof, a detector arranged for bias or anode circuit detection, in-

creases with increases in the strength of an ap-' plied signal.

Further. in accordance with the invention, a biasing circuit common tothe output or anode circuit of the detectorand the input or grid circuitof the amplifierisso arranged that the apparatus is, highly sensitive.to variations in the strength of the signalsapplied to the detector toprovide a more uniform output from the amplifier, the amplifier biasbeing automatically increased in apositive direction with increasedsignal strength applied to the detector.

The invention willhoweverbe better understood from the followingdescription when considered inwconnectionwith the accompanying drawing,and its scopewillbe pointedout in the appended claims; a

In thedrawing, Fig. 1 is, a schematic wiring diagram of an electricsignal amplifier and detector which may forma part of radio receivingapparatus and the like, provided withan automatic volume control circuitembodying; the invention, and Fig. 2 is a curve diagram illustratinganoperating characteristic of the, apparatus ofFignl. m I

. Referring to Fig. 1 of the-drawing,"5 is an electric discharge deviceof the hot-cathode, three-element type representing any'suitable deviceof this character for operation as a bias .detector of radio signals ormodulatedwaves. The detector is providedwith an input or grid circuit 6,an output-or anode circuit 7, and a cathode return-lead 8 common to boththe grid and anode circuits. suitable 'anode and grid bias potentialsfor the operation of device 5 as a bias detector are provided by asource of direct currentoperating potentials represented by avoltage-supply resistor '9,Lhaving positive and negative supplyterminalsrlO-IOas indicated, to which resistor the grid and anode.circuits fi and'l, and the cathode return. lead 8 are connected atsuitable tap points as indicated at 11, .12and 13 respec tively. s

As will-readily be understood, the above described connections are suchthat with a desired positive potential appliedxtotheanode circuit 7with'respect to the cathode, the grid circuit 6 is, provided with:au'negative potential with respect to the cathode such-that the anodecurrent: is reduced substantially to zero, with no modulated signal or.carrier.wave onathe grid'or input circuit, this beingv as is'wellvknown, a condition for properoperatiom of. device .5as1a; bias detector.Upon the application of signalavoltage to the input :ora gridlcircuitthrough "a suitable input device suchasiazh'igh' frequencytransfornier15, the. anodetjcurrent will increase in responseto increases in the;strength of the. carrier: wave. Modulated signals received :by thedetector, are transmitted'to outputterminalslfifrom the an!odecircuitthrough an output device such as a low or audio frequencytransformer 17. Amplified signal voltage is appliedto the detector .5thru asuitable. vacuum tube or, electric; charge amplifier havingsign'alinput terminals 18,

and an'output circuit 19 connected with the detector input or couplingdevice 15. The amplifier may'includejany desired number'of stages; Inthey present example, for the sake of simplicity and clearnessbut onestage is shown and includesan electric discharge amplifier device 20 ofthe type employed as a 'detectorat 5, and a couplingor in-- put device21interposed between the input terminalsxl8g-and an inputor controlvelectrode circuit 22 for:,device20. The anode orv output .circuitofdevice 20:,isprovided bythe.outputcircuit19' of the amplifier; to whichsuitable operating potential is applied through connections with thepoten-. tial supply resistor 9 at a tap point14 and through acathode"return"lead 23 for-the amplifier :con-,

hectedzwithithe resistor 9. at] a variable tap point24L p J Biaspotential for the amplifier grid or control electrode 1 is suppliedthereto through a lead" 25 connected with: animpedance suchas 1 aresistor 26in the;outputicircuit of the detector fi, to derive avariable :bias potential: therefrom responsive to; changes in thesignal-output." Thewalue of the potentialxapplieditolthei .controlelectrodeor. bias circuit of the amplifier through lead 25 is providedby an adjustable tap 2'7 on the impedance, which in the present exampleis arranged as indicated, to vary the voltage by varying the impedanceor resistance. The bias resistor or impedance 26 is preferably locatedas shown in the cathode return lead 8 for the detector 5 whereby itoperates to apply an additional bias to the detector in response toincreased signal input.

By returning the'control grid or input circuit of the amplifier 20 tothe positive end of the bias resistor 26 a variable positive bias orcontrol" potential, having a certainfixed initial value, is applied tothe control electrode circuit bythe detector device 5, in accordancewith the signal strength. It will be seen that if the variable tap point24 is brought to a point opposite the tap point 12, the total appliedbias potential to'the control grid of the amplifier device 20 will bethat provided by the drop in resistor 26 and will be a positivebias,'instead of a negative biasas is common with amplifier devicesheretofore used for signal amplification and the like.

The object of the above described arrangement is to provide a means forreducing the amplification of the amplifier by causing the biaspotential applied to the control electrode circuit, thereof to becomeincreasingly more'positive'with increased applied signal strength,thereby correspondingly reducing the amplification, to maintain theaverage value of amplification and the detector output substantiallyconstant.

With thetap point 24 for the cathode return lead of device 20 locatedsubstantially opposite the point 12, the detector through the'medium ofthe resistor 26 would supply all of the bias voltage for the'amplifierdevice 20. Should this initial bias, with no signal applied to thedetector, be too high or too low, the tap point 24 may be moved to'a'positionsuch as that shown in the drawing, along the potential supplyresistor 9.to provide the desired value of initial bias on amplifierdevice 20 for normal operation. In the present example, tap point 24 islocated at a point more positive than tap point 12 whereby the positivebias potential provided therebyis'applied in opposition to the positivebias potential supplied by the bias resistor 26. v

= The bias potential applied to the detector 5 is such that normallywith anelectric discharge device'of the typeshown, the anode current isvreduced substantially to zero or to a small fraction of a milliampere ashereinbefore described. The bias potential applied to the amplifier 20between its control grid and its cathode is then adjusted by applying asuitable potential to the cathode by moving the tap'point' 24 or the tap27. In the present example and in the usual case with electric"discharge devices of the type shown, the tap point 24-is moved to apoint more positive to balance or reduce the positive bias appliedthrough the bias resistor 26. The bias resistor 26 thus simultaneouslyapplies a negative bias to the detector and a positive bias to theamplifier.

It will be seen that with the above-described ar rangement the volumesetting of the amplifier depends upon the bias voltage applied theretoand may be adjusted by adjusting the resistor 26Tor the tap point 24.

It will be seen also that the circuit arrangement issuch that a positivebias potential'is applied .to the amplifier from resistor 26, while anadditional negative bias potential is applied to the detector overlthatderived from the supply source 9 between tap points 11 and 12.. Thisarrangement, in

accordance with the invention, provides for the proper operation of thenegative bias detector and for the operation of the amplifier with apositive bias in the steep portion of its grid-bias amplificavalue, theamplification falls off gradually, while as the amplifier and grid-biasvoltage is increased from zero to a more positive value, theamplification falls off-very rapidly. It is the purpose of theabove-described control arrangement to utilize this latter portion ofthe characteristic in the most effective positive bias control rangebetween points whereby a rapid and effective con trol of theamplification is obtained.

For the above purpose, when there is no signal applied to the amplifier20 and detector 5, the amplifier bias potential is so adjusted that theamplification is substantially at a point on the curve indicated at 29,the bias potential on the control grid of amplifier. device 20 beingthen slightly negative. When a signal or modulated wave is applied tothe input terminals 18, and through the amplifier 20, the bias potentialprovided by impedance 26 in the detector output or anode circuit willincrease in a positive direction which will cause the amplifier 20 to'operate on the steep portion of the curve such as at 30 for example. Theamplifier then operates on a portion of the curve which is moresensitive to bias voltage change than the negative slope and permits arapid reduction of signal strength when the latter tends to increase.

While but one amplifier device has been shown and described in thepresent example, it will readily be appreciated thatthe same type ofcontrol may be applied to other amplifier devices connected in cascadewith the single stage shown, substantially in the same manner as theconnection between the amplifier device 20 and the detector-5. In anycircuit arrangement, however, the amplifier bias control circuit isreturned through the impedance 26 in the detector anode or outputcircuit whereby the impedance 26 is arranged to apply a negative biaspotential to the detector and a controlling positive bias to theamplifier increasingly with increasing signal strength, the amplifierbeing operated at such a point on the characteristic curve that anincrease in the bias potential causes it to operate upon that portion ofits characteristic curve substantially between zero and a more positivebias voltage position, that is, on the steep slope of the positive biasportion of its grid-bias amplification characteristic curve as indicatedin Fig. 2.

A further advantageous feature of this volume control circuit is foundin the fact that the selective input device in connection withthe am- Ifor use in radio receiving apparatus and the like to which strong localsignals are applied and 0perates to improve the fidelity under suchconditions while for other signals it operates to increase theselectivity. Stated in another way,

as the bias potential on the control grid circuit upon the input deviceand thereby decreases the selectivity and increases the fidelity.

The control arrangement of the present example is particularly adaptedfor use in connection with the second detector of a superheterodynereceiver, the latter being supplied with a signal voltage through themedium of an intermediate frequency amplifier. For example, amplifierdevice 20 may be consideredto be the last amplifier device of anintermediate frequency amplifier, and device 5 may be considered to bethe second deector in a superheterodyne receiving apparatus.

From the foregoing description, it will be seen that an amplifier anddetector circuit embodying the invention provides a rapid and effectivecontrol of the output volume from the detector through the medium of abias impedance controlling the applied grid potential to the precedingamplifier, the grid potential of the amplifier being controlled in apositive range of potentials having an amplification characteristicwhichis sharply sloped in a short range of positive potentials,substantially as indicated in Fig; 2, and that this arrangement permitsthe use of a highly plifier without sacrificing the quality of theoutput in response to strong or local signals.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is V 1. In combination, an amplifier having an anode, a cathode,and a grid, a detector having a I grid-coupled to said anode of saidamplifier, a cathode, and an anode, a common source of operatingpotential for said anodes, a resistance connected between the cathode ofsaid detector and a point on said source negative with respect to bothof said anodes, and connections from said resistance to the grid of saidamplifier and from said source to the cathode of said amplifier, the

potential of said resistance between said connections thereby beingapplied in a positive sense to the grid of said amplifier.

2. In combination, an amplifier having anode,

cathode, and grid electrodes, a detector having a grid electrode coupledto said anode electrode, a cathode and an anode electrode, a commonsource of operating potentials for said amplifier and detector,connections from all of said electrodes to said common source ofoperating potentials, said connectionsfrom the grid of saidamplifier'and cathode of said detector to said source including a commonresistance whereby the potential on said resistance produced by anodecurrent insaid detector is supplied in a positive sense to the gridofsaid amplifier and in a negative grid of said detector.

WENDELL L. CARLSON.

sense to the

